Usually, noble metals show drastic changes in their electronic properties at nanoscale. The electronic properties of noble metals become more interesting when the size decrease to the extent where it would be comparable to the Fermi wavelength of an electron. Such small size regime is called metal clusters, where the free electrons are confined relative to the Fermi wavelength in the conduction band. These metal clusters composed of few atoms to few tens of atoms and exhibit molecule like, size dependent properties where the valence and conduction bands show further discritization. Excitation of electrons from valence band (filled 5d10) to the conduction band (6sp1) leads to a strong size dependent fluorescence emission in a range that can extend from the visible to the near-infrared. The synthesis of Au QCs with different core sizes have been reported recently, where the mass spectroscopy, especially, MALDI-TOF was ideally used as a major characterization tool.
In the past decade, researchers have reported multiple step synthesis of different core size of Au QCs employing various capping and reducing agents where most of them are either bulky (like protein or polymers) and/or non-biocompatible which limits their application in biological fields. While Au has been tried, Ag being less stable, gets oxidized and therefore is not very attractive for applications similar to Au.
In other prior arts, alkane thiols are used, which is not recommended, especially for medical applications.
The assembly of gold nanoparticles in presence of semi-essential amino acid such as cysteine for biomedical applications is reported in the art.
Aurora Mocanu et al. in Colloids and Surfaces A: Physicochemical and Engineering Aspects vol 338, 1-3, April 2009, Pgs 93-101 discloses preparation of gold nanoparticles in aqueous dispersions, using sodium citrate as reduction agent, and their interaction with L-cysteine. Further preparation of citrate capped gold nanoparticles of controlled size in aqueous solution and their interaction with L-cysteine is reported by Petean I. in Journal of optoelectronics and advanced materials; 2008, vol. 10, no. 9, pp. 2289-2292.
The gold nanorods conjugated with I-cysteine and their applications” is disclosed by M. M. Dzaglil in the African Review of Physics (2012) 7:0055 475. (Additionally 1H MAS NMR study of cysteine-coated gold nanoparticles is described in J. Phys. Chem. B, 2012, 116 (27), pp 7771-7775 by Anuji Abraham.
Further KR20040060357 discloses a substrate for biosensor and a method for preparing thereof. The substrate for biosensor comprises a solid substrate with one side covered with a metal thin layer; a self-assembled peptide on the metal thin layer of the solid substrate through the cysteine residue in the end; a protein bound to the peptide and having specificity to the invariable region of an antibody; and the antibody bound with the protein, wherein the metal thin layer is composed of gold, silver, copper or white gold; the peptide consists of 5 to 25 amino acids; and the solid substrate is treated with mercaptoethanol solution or mercaptopropionic acid solution.
Zheng and Ying in J. AM. CHEM. SOC. 2009, 131, 888-889 titled “Protein-Directed Synthesis of Highly Fluorescent Gold Nanoclusters” disclose gold nano clusters of a single size, ˜25 atoms, which emit red fluorescence, prepared by using HAuCl4, NaOH and a large protein, Bovine Serum Albumin (BSA). Further, the process is a 12 hour reaction to synthesize the nano clusters.
Currently the challenge lies in providing a process to synthesize water dispersive, biocompatible and a range of Au QCs core size population with different emissions, especially, using small, biocompatible non-bulky molecules. Actually, the tuning of the size of Au QCs core, specially using small biomolecule, is a very difficult and quite tricky task.
Moreover, there is no report on the synthesis of small non-bulky and non-essential amino acid molecule conjugated Au QCs so far, wherein amino acid itself provides the site for sequestration of gold ions and plays the role as both reducing and stabilizing/capping agent. Therefore cost-effective, industrially viable and environmental friendly process for the synthesis of water dispersible, highly biocompatible and fluorescent non-bulky amino acid capped or conjugated metal quantum clusters with a range of core size for biomedical applications is highly desirable, which obviates use of any toxic materials.